Remove "aValue" param

Since it's now always clear when we have an array of args (because the
types were recently refactored to use IRubyObject[] rather than an
IRubyObject that was sometimes a RubyArray), we can delete the flag
that was being used to track whether given arguments should be treated
as an array or not.
Merge pull request #1232 from dmarcotte/remove-is-array

Remove now-unneeded "aValue" param from yields